In the field of electrical distribution, it is useful to be able to measure current flowing through conductors. Often these conductors are operated at high electric potential relative to ground and thus it is advantageous to make the measurement without direct electrical connection.
It is possible to make such measurements using a current transformer. Such devices use a magnetic circuit to couple the current in the conductor to be measured (primary conductor) and the current in a secondary conductor. This magnetic circuit typically consists of a yoke of ferromagnetic material having a conductive coil (the secondary conductor) wound therearound.
There are a number of drawbacks to this kind of current sensor. The magnetic circuit may saturate either as a result of impedance in the secondary circuit, the presence of a non-time varying current in the primary conductor, or the presence of an externally imposed magnetic field. Any of these conditions results in the current sensor losing accuracy. Furthermore, hysteresis in the magnetic circuit leads to non-linear sensor response at low operating current.
Most modern current sensors can be constructed using surface coils uniformly spaced around a central cavity through which the primary conductor passes. These are known as Rogowski sensors. Rogowski sensors include an air core, rather than a magnetic core. Surface coils include one or more turns of a conductor provided on a substrate, for example by etching or using printed circuit board technology. In order to obtain a good rejection of magnetic fields that are not due to the primary conductor, the surface coils are very precisely aligned on axes radial to a central axis of cavity through which the primary conductor passes. Such a sensor is described in U.S. Pat. No. 6,965,225 B2.
FIG. 1A is a schematic of a current transformer 10 such as is known in the art. The current transformer 10 is interchangeably referred to as a sensor arrangement 10 in the following. The current transformer 10 comprises a plurality of sensor elements 102-1 to 102-6 arranged around a central point through which the primary conductor 100 passes when the current transformer 10 is in use. Each of the sensor elements 102 comprises one or more turns of a conductive material 104 provided on a substrate 106. A single sensor element 102 is shown in FIG. 1B.
The substrates 104, and thus also the sensor elements 102, are aligned radially with the central point. Each of the sensor elements 102 is configured such that when a magnetic field passes through it a current (also referred to herein as a signal) is induced in the sensor element 102. The signal induced due to a magnetic field passing through in a first direction is positive, while the signal induced by a magnetic field passing through in a second opposite direction is negative. The plural sensor elements 102 which of which the sensor arrangement 10 is comprised are arranged such that a circular magnetic field due to the primary conductor causes signals of the same sign to be induced in each of the sensor elements 102. As such, the signals from each of the sensor elements 102 sums with the signals of the other sensor elements 102. In FIG. 1A a current is flowing through the primary conductor in a direction into the page, which causes a circular magnetic field in a clockwise direction (which is denoted by the dashed circle and arrows).
Sensor arrangements such as that shown in FIG. 1A exhibit good far-field performance (i.e. a high level of rejection of fields not due to the primary conductor) due to the precise radial alignment of the sensor elements, the equal spacing of the sensor elements around the centre, and the fact that each sensor arrangement has the same configuration as the others.
U.S. Pat. No. 6,965,225 B2 describes a sensor arrangement similar to that of FIG. 1A. The sensor arrangement of U.S. Pat. No. 6,965,225 B2 is adapted so as to be retro-fittable to an existing conductor. This involves a mechanical hinge system whereby the sensor elements can be temporarily separated so as to allow the primary conductor to be passed between them.
The present invention provides a retro-fittable sensor arrangement which can require no moving parts and therefore be cheaper to manufacture and less prone to mechanical failure than this prior art arrangement.